In a shaded pole motor, there typically are a number of individual magnetic laminations stacked on one another and secured together to define a stator frame having a central opening for receipt of a rotor. The stator is shaped to define a number of circumferentially spaced poles which terminate in radially spaced relationship to the rotor and the poles have windings thereon formed by a wire wound around the pole which must be insulated from adjacent turns of the wire and from the laminations. The adjacent windings are connected typically in a crossing fashion to simultaneously generate opposite polarity when connected to an alternating current power source, and the number of poles determines the operating rotor speed. For alternating with 60 cycle altnerating current a 2 pole motor would operate at approximately 3,600 rpm; a 4 pole motor would operate at approximately 1,800 rpm, and a 6 pole motor would operate at approximately 1,200 rpm. A shunting conductor or shading loop is typically located near one end of each pole face and delays the magnetic buildup along the face, thereby providing a unidirectional rotation to the rotor. The rotor is typically without any windings but is fabricated of a conductive material having its surface periphery formed as axially extended and separated raised land areas and grooves.
In such shaded pole motors, one form of insulation used is a paper-like sleeve or form which is rested against the laminations of the stator and projected beyond the ends of the stator, and the field wire is wound around these insulating sheets and is thereby maintained separated from the stator. This necessitates a minimum given clearance between the winding and the stator, such as a quarter of an inch, so that the diameter of the winding must be approximately half an inch larger than the width of the stator plates.
There currently are epoxy coatings which can be adhered to the field stack, and the Underwriters Laboratories (UL) approves this type insulation if there exists a minimum thickness of approximately five thousandths of an inch (0.005 inch) on the flat and/or on the corners. The field winding can be wound directly over the coated field plate stack thereby reducing the size and cost of the winding coil, where the same wire is used with the same number of turns, as compared to the stator noted above.
This epoxy coating typically does not operate satisfactorily where the shading loop is located, since exterior cracks or gaps form between the shading loop and the back face of the field stack and between the shading loop and windings to reduce the effectiveness of the insulation below acceptable standards. Epoxy coating typically is not successful to close internal cracks, since even though it might appear full and continuous after a period cracks or pin hole breaks appear. If a very thick layer of epoxy is laid over the area of the shading loop, this increased cost of the epoxy nearly always offsets any decrease in the winding costs.
Many processes are available for coating motor parts with epoxy, but few are successfully used with shaded pole motors. It is possible, for example, to electrostatically charge the motor field plate stack and the fine powdered epoxy with opposite polarity and to apply the powder uniformly onto the stack in any number of ways, such as by exposure in a cloud chamber created by bubbling air through a bed of powder, or by various spraying techniques using venturi draw from a powder bed. After the motor stack is so powder coated any area where fine tolerance is required, such as on the inner and outer faces of the stack or in bolt holes, the same can be vacuumed clean. The powder coated stack is then heated in an oven to an appropriate temperature for a proper duration to set or cure the epoxy. It is also possible to coat the motor parts heated already to the setting temperature, where the particular powder epoxy is applied to the heated stack much in the same manner as noted above, but any area that is to remain uncoated must be masked first before exposure to the powder.
It should be noted that one of the pecularities of coating with an epoxy by almost any known technique is that the epoxy does not cover small cracks or interior areas effectively since there is a tendency for voids and the like to appear even after the coating is heat set.
By contrast, the epoxy can be applied quite uniformly over continuous or tightly innerfitting shallow corners even on adjacent members, or can fill in rather large gaps or voids effectively. In a sense, this invention utilizes this particular characteristic of the epoxy for improving the very nature of epoxy covered motors, and can be understood and appreciated more completely by referring to the detailed description of the invention which follows.
Specific prior art patents which might be of some assistance in teaching the various aspects mentioned above include: Walters U.S. Pat. No. 1,838,122; Moehlenpah U.S. Pat. No. 2,886,722; Jordon U.S. Pat. No. 1,823,979; Morrill U.S. Pat. No. 3,158,769; Swain U.S. Pat. No. 3,780,323; and the Great Britain Pat. 759,960.